In the fabrication of teeth for cutters and especially teeth to be used in drill bits for drilling oil wells, one desirable form is a composite tooth. It is formed of a bonded polycrystalline diamond compact (PDC) layer on a hard metal substrate serving as a support. The stud or substrate for the PDC is normally constructed of tungsten carbide metal which provides an unusually hard metal body. Tungsten carbide in particulate form must be molded and typically is supported in a matrix of some suitable base metal to form a support readily anchored as a stud. While the supportive stud can be shaped as an elongate cylindrical body, an end face is constructed at a right angle, or it may be a sloping face such as a chisel. As a generalization, tungsten carbide is the best and hence the preferred material for assuring that the support is able to withstand substantial impact in operation as a cutting tooth. It is possible to use alternative embodiments such as other types of carbides which are bonded into the support.
The present disclosure is directed to an apparatus and method for making a support with a polycrystalline diamond compact (PDC) layer on the end of it. While the preferred material is polycrystalline diamond compact, an alternative material is boron nitride. The first preferred material is polycrystalline diamond compact and boron nitride is second, and they are identified by the symbols PDC or BN respectively.
The process of joining the PDC to the support (primarily tungsten carbide or WC) involves brazing. One such procedure for this is in U.S. Pat. Nos. 4,319,707; 4,225,322; 4,772,294 and 4,527,998. They disclose a method and apparatus for forming a brazed diamond surface on the end of the support. One form of apparatus utilizes a water cooled system. One reason for this water cooling is the amount of heat liberated during the brazing operation. The large amount of heat liberated almost destroys the materials of the finished product. As will be understood, there are maximum limits on the temperature, and especially the temperature achieved by instantaneous melting of the braze material joining the PDC layer to the support. As set forth in U.S. Pat. No. 4,319,707, an RF source causes inductive heating. Heat is generated and flows readily into the WC component as well as the diamond component. So much heat is liberated that a water cooled system is used. As detailed especially in FIG. 4 of that disclosure, a water cooled coil must be used to limit the RF generated heat that is liberated in the work piece.
The present disclosure sets forth an enhanced manufacturing process and the related equipment for manufacture of a diamond coated member, namely, a PDC covered support. In this instance, the sandwiched components are first assembled by positioning the support and the PDC in a spaced relationship. The two blanks are separated by an adhesive metal layer. The metal layer is formed of a selected material which responds to brazing. The metal however is selected so that the maximum temperature required in brazing is less than the temperatures which destroy the WC, but it may be high enough to destroy the PDC components. In other words, temperature limits are imposed on the brazing process to avoid product damage and destruction. Without observation of practical temperature limits, the entire product can overheat, thereby destroying the integrity of the joined components by exceeding permitted temperature levels. For both the PDC and WC of the fabricated tooth, there are practical working maximum temperatures. The peak temperature actually achieved in the adhesive metal during the process determines the peak temperatures that are experienced in the PDC or in the support. Limiting the peak temperature is made somewhat more difficult because PDC is a very good heat conductor. In other words, any heat liberated in the braze material is conducted out of the metal material into the PDC blank and may damage the PDC blank if the peak temperature is excessive. To escape this, the present disclosure sets forth an apparatus which provides for cooling of the PDC support. It is not adequate merely to provide a heat sink on the PDC layer as in the prior art. Indeed, it also is not adequate merely to conduct water through a cooling jacket or other facility which is remote from the PDC layer. Rather, optimum cooling has to be against the support to assure that the joined PDC and support do not become sufficiently over stressed by exceeding the tolerated or permitted temperature.
The PDC layer is a very good heat conductor. The present disclosure contemplates cooling of the support rather than the PDC layer so that there is no need to transfer heat through PDC, a component which can be easily thermally damaged. By way of contrast, U.S. Pat. No. 4,319,707 shows a water cooling system in a supportive head. While that may do a very good job of keeping the head cooled, the heat transient that occurs with RF heating is brief, and is especially fast in light of the conductivity of the PDC material. Restated, the PDC material is readily able to conduct heat so damage from overheating is possible. The PDC disk comes in the form of a round, normally cylindrical button. The button is exposed directly to the cooling material flow. The detriment to this mode of manufacture is that the PDC disk must normally be contacted by a supportive head. Prior art systems such as shown in U.S. Pat. No. 4,225,322 require that the PDC disk be contacted with, resting on a ceramic support. The head has, in the preferred embodiment, a number of passages which carry a gas directly against the PDC disk is held in the head, being positioned in the head, as an insert. The disc does not need to be gripped or tightly held; rather, it needs to be located for exposure to the gas flow which provides a neutral or slightly reducing atmosphere to prevent PDC degradation. In the preferred embodiment, the coolant is a flow of chilled reducing or inert gas. More specifically, the gas is cooled or pressurized so that the heat content of the gas stream is markedly reduced, almost to zero.
Methods in the prior art devote substantial effort and attention to maintain contact between the PDC disc and the heat sink to thereby prevent damage to the PDC disc. Often however, this contact is not accomplished and damage still occurs because of shifting or warping of the PDC disc which often occurs in conjunction with irregular melting of the brazed layer (typically a foil) abutting the components.
The present system has a substantial advantage as a result of direct gas flow against the PDC disc and heat flow out of the disc. By operating in this fashion, the heat which is experienced at the PDC disc is carried away quickly so that damaging maximum (or peak) temperatures are not achieved. By distributing a number of passages or ports in the head, sufficient flow paths for directing the gas flow against the PDC disc are then provided so that heat is reduced before damage is inflicted on the disc. The heat is evenly removed at a controlled rate due to uninterrupted contact of the support and the metal members in contact with it.
In one aspect of the present disclosure, a press is set forth which assembles the unfinished components in a desired alignment. This involves a lower head formed of stainless steel which has a recess or cavity to support and hold upright the PDC support formed of WC or other hard materials. It is a relatively stable, fixed, preshaped structure which is able to maintain shape and hardness through the temperatures which are achieved in the present fabrication procedure. It is able to withstand temperatures in excess of about 1300.degree. C. without damage. Moreover, the presently disclosed apparatus is used to align the lower head with the upper head so that the tooth part of WC or similar hard materials is able to support and receive a layer of adhesive material which is melted by induction heating through the use of an RF coil. While the heating interval is short because the mass of the adhesive material is relatively small, it is possible nevertheless that the temperature will spike high, creating a tremendous heat flux. The adhesive layer is clamped between the supportive hard metal insert below and the PDC disc above. While both are relatively good heat conductors, the PDC disc is a much better heat conductor. The present disclosure accommodates this aspect which enables excessive heat flow into the support, and is able to cool the PDC disc so that a proper bond is formed at one face of the PDC disc while the body of the PDC disk is cooled by flowing heat into the support and its contacted members thereby preventing damage by spiking to an excessive temperature. The present system thus provides a manufacturing system for fabrication of a brazed PDC disc on a support formed of hard metal, and also provides a procedure whereby such inserts can be fabricated.
An important aspect of the present disclosure is the ability of the system to fabricate numerous units without building up a residue of heat in the components of the equipment which require cooling for an interval thereafter. Rather, cooling is accomplished during each operative use of the equipment. The system is able to accomplish brazing with braze materials which do not cause excessive heating to make an adequately brazed joint, without excessive temperature spiking which would otherwise damage the sandwiched components forming the composite tooth or insert.
However the prior art as represented by U.S. Pat. No. 4,225,322 warns one not to exceed about 700.degree. C. The reference points out that thermal degradation begins for the PDC layer at about that temperature. While this statement is technically correct, in two subsequent patents by the same inventors, they cite a diamond layer peak temperature of 840-900.degree. C. which is excessive and detrimental to the PDC. Reference is made specifically to U.S. Pat. Nos. 4,527,998 and also 4,772,294. It is noteworthy that the presently disclosed invention maintains the peak temperature of the PDC layer under a 700.degree. C. at all times during the process. To the extent that higher temperatures exist, they occur in other components, not the PDC layer, and temperature damage is thereby avoided.
One aspect of the present disclosure is that the system is able to operate with a variety of sizes by utilization of different sized metal supports. This system also creates a reducing atmosphere at a braze joint area without the need to reduce or evacuate a large chamber.
This invention uses a blank which is 13.7 mm in diameter and 4.00 mm thick.
General Electric uses a 2530NC diamond blank which is 13.3 mm in diameter and 3.53 mm thick. There is a difference of approximately 90.degree. C. in the performance of the two sizes during fabrication. It has been discovered that there is a relationship between the thickness and diameter of the PDC disc. The discs are blanks preferably in the following relationship correlating thickness to diameter. The relationships are (given in mm):
______________________________________ thickness diameter ______________________________________ 4.0-5.0, 7-11, 4.0-5.5, 11-15, 5.0-9.0, and 16-20, and 8.0-10.0 20 and greater ______________________________________
The table of correlated blank thickness to diameter given above operates in conjunction with the maximum temperature interposed on PDC bonding to thereby prevent high temperature spikes in the diamond layer. Substantial problems arise from the heating of the PDC blank. It functions as a simple bimetallic strip with a high residual stress stored as a result of quenching at high pressure and temperature. The residual stress is beneficial during rock cutting operations because the PDC component is maintained under compression during use. Taking into account the inherent thermal expansion of the underlying substrate (typically formed of tungsten carbide in a cobalt matrix), the resultant stresses from thermal expansion during this in conjunction with the compressive impact loading provides a PDC disc which is very long lasting in the rugged conditions in which it is normally used. Referring back to this table, if the disc is either too thin or the ambient operating temperature is excessively high, or an undesirable combination of both, cracks can be formed by catastrophic stress relief during operation. This occurs in the ongoing presence of high reactant ambient chemical exposure including those that enhance oxidation or other surface destructive mechanisms. Other mechanisms in addition to oxidation include delamination, graphitization, and stress cracking.
One aspect of adhering to the values set forth in the foregoing table is that the production scrap rate is marketedly changed. While the scrap rate can be as much as ten percent in the practice of the process shown in the GE references mentioned above, and perhaps as low as five percent depending on operating skills, the process set forth in this disclosure has a scrap rate of only about one percent. Substantially all of that loss can be directly attributed to operator error. Moreover, the completed product is substantially more desirable in every regard.